Essentially all ion exchange resins are presently produced from crosslinked copolymer particles or "beads" by chemically treating the same to attach or form functional groups therein having a capacity for ion exchange. Thus, copolymer beads provide the strong, insoluble and rigid substrate for carrying ion exchange functional groups. From a standpoint of durability and hydraulic characteristics, an ion exchange resin is no better than the crosslinked copolymer from which it is derived. Highly porous copolymer particles without functional groups (herein termed "macroreticular" or "macroporous" polymer) are also useful as adsorbents for removing organic materials from fluid mixtures thereof and are intended to be embraced within the meaning of "ion exchange copolymer particles".
As used herein, the terms "functionalize", "functionalized" or "functionalization" are intended to describe the known prior art of chemically treating an insoluble crosslinked copolymer bead to attach an ion exchange group (functional group) thereto. The copolymer bead serves as the backbone polymer, whereas the ion exchange moiety is the active or "functional" site capable of exchanging ions with a surrounding fluid medium. Of the strongly acidic cation exchange resins, the sulfonic acid resin formed by sulfonating a copolymer (e.g., with sulfuric acid) is perhaps best known in the art. Weakly acidic cation exchange resins are normally derived from crosslinked acrylic copolymers by merely hydrolyzing the copolymer to form carboxylic cation exchange groups. Chloromethylation and amination of styrenic copolymers will result in weakly basic anion resins or strongly basic anion resins. Methods of performing suspension polymerization of ion exchange copolymers and of functionalizing the same to ion exchange resins can be found in the prior art and, in particular, reference is made to U.S. Pat. No. 4,224,415, which reference is hereby incorporated herein by reference.
Historically ion exchange copolymers have been formed by a batch process using a kettle reactor for monomer droplet formation and polymerization of an aqueous suspension of said monomer droplets. The monomer droplets are formed and maintained in a suspension by use of a mechanical agitator in the kettle. As might be expected, mild agitation forms monomer droplets (and eventually copolymer beads) of relatively large size while vigorous agitation yields smaller droplets. In either event, agitation of this type invariably leads to beads having a wide distribution of copolymer particle sizes. Ion exchange resins derived from copolymers of widely differing sizes are, in turn, of widely differing sizes. For many applications the wide distribution of bead sizes is not a major problem. For other applications it is desirable to have uniform bead sizes.
To accommodate those uses where narrow distribution of resin size is preferable, most manufacturers mechanically screen either the copolymer beads or the ultimate ion exchange resins to eliminate "unders" and "overs", that is, fines and oversized beads. Unfortunately, it is difficult with present technology to screen wet beads, and drying of the beads is not otherwise required for most purposes. The loss of product yield is another reason militating against screening resin beads. Accordingly, there is a definite desire for a new commercial process to furnish uniformly sized copolymer beads that can be functionalized to have ion exchange properties.
The prior art discloses several methods of growing larger particles from smaller seed particles. Among these can be found methods for (1) growing styrenic or ethylenic polymer particles by feeding monomers into an aqueous suspension of particles having the same composition, (2) swelling of preformed styrenic polymers or copolymers with liquid monomers (in situ) followed by suspension polymerization of the swollen particles and (b 3) swelling of minute low molecular weight emulsion particles by inbibition of monomers (and optionally solvents) in the suspension.
Illustrative of the known techniques utilizing seed to grow larger particles under aqueous suspension conditions, is Canadian Patent No. 890,463 (Sekisui/1972). Specific examples show uncrosslinked polystyrene and styrene/acrylonitrile copolymers used as seed and styrene or styrene/methyl methacrylate as the imbibed monomers. Continuous or intermittent addition of monomers over a three to twelve-hour period is illustrated. The reference also teaches the requirement for an expanding agent (foaming agent). Crosslinked seed is not exemplified.
Further refinements of the process of feeding monomers to a suspended styrenic seed may be found in subsequent patents assigned to Sekisui Kagaku Kogyo Kabushiki Kaisha. Among these are UK No. 1,416,405 (1975); UK No. 1,418,560 (1975); U.S. Pat. No. 3,959,189 (1976); U.S. Pat. No. 4,085,169 (1978); and U.S. Pat. No. 4,091,054 (1978). These later references teach the use of screening techniques for the seed in order to produce a uniform styrenic bead and also variations in the techniques of adding the monomers and the catalyst mixtures. In one of the references ('560) the improvement involves placing the catalyst in a separate feed stream from the bulk of the monomer mixture and utilizing a solvent with the catalyst. Seed particles of polyethylene are swelled with styrene in another of the patented processes ('179). Although the various Sekisui patents disclose combinations of monomers in both the seed and the monomer mixtures and even allude to the use of crosslinkers, clearly none of the teachings describes a method for producing crosslinked styrenic or ethylenic copolymers utilizing a feed containing substantial amounts of crosslinker (as needed for ion exchange copolymer resins). In the examples of the references either the seed particle is a homopolymer of styrene, ethylene, or the like, or the monomer feed is comprised of a single monovinyl monomer, or both. As will be explained more fully hereinafter, the use of a polyethylenically unsaturated crosslinking monomer, especially in large amounts and highly reactive types (such as divinylbenzene), presents unusual and difficult problems in maintaining a suitable aqueous suspension.
Other techniques for imbibing monomers into a preformed suspension of particles include the formation of the so-called "hybrid" resins which are produced when absorbing the monomer mixture into the macropores of a macroreticular resin, thereby forming two discrete phases within a single particle bead (see e.g., U.S. Pat. No. 3,991,017). Unlike the Sekisui technique which comprises the growing of particles to larger size, the hybrid resins largely imbibe the monomers into voids or spaces within the particle as well as into the gel matrix of the particle itself and thereby limited swelling is normally accomplished. In order to imbibe a polyethylenically unsaturated crosslinking agent into a preformed liquid monomer mixture containing a monoethylenically unsaturated monomer and a crosslinker, a very carefully controlled suspension system is required (see U.S. Pat. No. 3,792,029). By this method monomer droplets are formed containing both a styrenic monomer and a crosslinking agent and thereafter an emulsion containing additional crosslinker is fed to the suspension to make up for the loss of the faster reacting crosslinker while balancing the stabilizer amount to prevent beads from agglomerating (coalescing). Although differing from the prior art techniques utilizing an initial suspension of seed particles, the process of the '029 patent recognizes many of the problems associated with avoiding a new population of fine particles when feeding monomers during polymerization.
A second group of prior art references teaches methods for imbibing monomers into preformed particles to swell the same, and subsequently polymerizing the swollen particles. U.S. Pat. No. 3,332,890 (1967) is an early reference showing the manufacture of "snake-cage" resins produced by imbibing monomers into a styrenic gel polymer to form a linear polymer within the crosslinked copolymer bead. The process involves soaking crosslinked copolymer beads with a monomer mixture and thereafter suspending and polymerizing the swollen beads. Other typical processes for swelling particles or beads prior to polymerization can be found in Romanian Patent No. 48091 (1967) and UK Patent No. 1,116,800 (1968). A variation wherein the monomers soaked into the bead comprise both mono- and divinyl monomers is illustrated in UK Patent No. 728,508 (1955). A so-called double polymerization process is taught in U.S. Pat. No. 2,960,480 (1960).
In more recent years investigators have shown the feasibility of growing larger particles from a seed under emulsion polymerization conditions. See for example U.S. Pat. Nos. 4,113,687 and 4,186,120 (also European patent application Nos. 3905 and 10,986, as well as UK Patent No. 1,527,312). By this emulsion process, growth of the particles is propagated by maintaining conditions such that molecular weights of the polymers remain low or by utilizing selected solvents which can swell the particles.
A copending patent application (U.S. Ser. No. 393,953, filed June 30, 1982, now U.S. Pat. No. 4,419,245) commonly assigned to the assignee of the present application generally teaches a process to produce crosslinked copolymers of high mechanical strength and preferably narrow particle size distribution by the gradual addition of monomers to a suspension of seed particles maintained under polymerization conditions. This process comprises: (a) forming an unprotected agitated suspension of lightly crosslinked styrenic or ethylenic copolymer seed particles in an aqueous phase; (b) feeding to said unprotected seed suspension at least one polyethylenically unsaturated styrenic or ethylenic crosslinking monomer and a monoethylenically unsaturated styrenic or ethylenic monomer, which monomers are imbibed by and swell the seed particles without agglomeration of the particles in the suspension system; (c) continuing the monomer addition to the suspension until the seed particles have imbibed an amount of monomer sufficient to swell the particles to the desired size (but insufficient to stress the particles to a point of fracturing upon subsequent functionalization or to cause particle agglomeration or the growth of a new fines population within the suspension); (d) terminating the polymerization subsequent to the feeding step when the imbibed monomers have been sufficiently polymerized (e.g., 95-99% polymerized); and (e) separating the resulting insoluble copolymer particles from the aqueous phase. The term "unprotected" means that the dispersion is largely free of the usual protective colloids. When the copolymer is to be used for ion exchange, the monomer mixture will normally contain mainly a monoethylenically unsaturated monomer (e.g., 98-80%) with the remainder crosslinking monomer. As used herein "monomer mixture" is intended to refer to combinations of different monomers whether physically admixed or not during addition to the seed particles.